With an electronic gas control valve

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If the pilot flame goes out, it may take up to 180 seconds for the thermocouple to cool enough for the solenoid to deactivate and close the relief valve.
The relief valve determines whether gas can enter the gas control. If the relief valve is open, the gas control operates normally, continuously releasing gas into the pilot flame through the pilot gas control valve and supplying gas to the burner through the main control valve when the thermostat sensor detects the water temperature is below the set point selected on the dial located on the front of the gas control.
How to Test a Thermocouple

The pilot flame heats a thermopile instead of a thermocouple. The thermopile works on the same principle as a thermocouple, except that it is larger and creates more current than a thermocouple, enough current to power the circuit board inside the gas control valve. Unlike a thermocouple, current from the thermopile passes to the gas control valve through two japan telegram data wires that connect to the front of the gas control. The printed circuit board controls a series of self-test procedures, compilation of water heater operation errors, etc., and uses solenoids to control the flow of gas that enters the gas regulator and the gas that goes to the pilot and burner. See Main Parts of a Gas Water Heater

If current is not produced by the thermocouple or thermopile, apply gas to the valve until the pilot is re-ignited or the faulty thermocouple or thermopile is replaced.

The thermostat for reading the temperature on the gas valve is located inside the copper tube that protrudes into the tank and reads the water temperature. When the water temperature drops below the selected thermostat setting, then the gas valve releases gas through the manifold tube and into the burner located in the combustion chamber.